instance FromRow IIM where 
  fromRow = IIM <$> field 
                <*> field 
                <*> field 
                <*> T.fromPGArray  (fmap T.fromPGArray field)

Which I expected to work based on the fact that this is what I need to convert (PGArray (PGArray Double)) to [[Double]].

Why is there an additional fmap needed?

On Saturday, December 27, 2014 10:23:47 PM UTC+11, Tom Ellis wrote:

On Fri, Dec 26, 2014 at 09:31:02PM -0800, in...@rotnetix.com wrote:
> I would actually like my data IIM to be:
>
> data IIM = IIM {key :: String
>                ,itype :: String
>                ,idet :: Maybe String
>                ,imat :: [[Double]]} deriving (Read, Show, Eq)
>
> But I can't figure out how to get a fromRow instance for that

instance FromRow IIM where
  fromRow = IIM <$> field
                <*> field
                <*> field
                <*> fmap (T.fromPGArray . fmap T.fromPGArray) field

> PS. I started looking into Opaleye as a possible DSL and would appreciate
> any help you can give me in figuring out how to represent arrays in that.
>  I am assuming I need to do something with queryRunnerColumn but I was not
> able to understand the example.

This is indeed somewhat fiddly to do because it requires fiddling with the
implementation of postgresql-simple instances.  Unfortunately that library
does not provide us with enough primitives to do this directly.

I will work on this.

Tom

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